Recording the sky as earlier observers did

This lesson asks you to locate features of the night sky with star charts, computer models or direct observation, and to record their position in astronomical terms and systems. The museum cannot do the locating for you. It offers records in which earlier observers wrote down what the sky showed, so you can compare their way of recording with yours. ¹

Ask what a later reader would need in order to find the same feature again: where, when, how long, and from which place.

Compare the words of each record with the terms of the expectation, such as the celestial equator and the ecliptic, and list what is missing before you could plot the feature on a chart.

Read a record of a single sentence: in 905 a tailed star rose from the east of Najd; its rising was in Aries, its tail over Yemen and its course through Syria ². The sentence gives a place of rising, a sign, a place for the tail and a direction of travel; the record of the star gives no day of the year, no altitude and no azimuth. The record of eclipses says that al-Farghani states that if the conjunction falls away from the nodes, parallax makes the visible differ from the true, and an eclipse may appear partial from one place and total from another ³⁴.

The museum's records hold no statement in which al-Farghani says why he wrote on eclipses, and they do not supply a reason for him. What they show is this: al-Mas'udi counts al-Farghani among those who engaged in the sciences of the heavens ⁵; Golius places his time in al-Ma'mun's era and notes that the author mentions al-Ma'mun's observatories in chapters five and eight ⁶; and the chapters show al-Farghani estimating the earth's circumference from a degree measured by the cubit in al-Ma'mun's days ⁷. His motive for this part of the book is not answered in these records.

The tailed star reaches us in one sentence, which the record takes from Shadharat al-Dhahab, page 442 ²; the record does not say when the report was written or from whom it was taken. Al-Farghani's text is read in a Latin edition printed at Amsterdam in 1669, titled "Elementa astronomica" ⁸. The eclipse record estimates the length of the earth's shadow-cone in Ptolemy at about 268 earth-radii and says these figures are OCR readings and are doubtful ⁹. Among Golius's examples is a lunar eclipse Ibn Yunus observed on the night of Thursday in the year 368 of the Hijra; the eclipse's diameter was more than eight fingers and less than nine ¹⁰. The Diocletian year is printed as "69" where 694 is correct by the other reckonings, a correction flagged, not relied on without checking ¹⁰. For each observation, ask who saw it, who passed it on, and in which edition it reached us.

Al-Farghani states that an eclipse may appear partial from one place and total from another. Suppose two historians in two different places described the same eclipse, one as partial and the other as total. Take a position on whether one of them must be wrong, and answer the strongest objection to it: is it not the rule that two honest witnesses describe the same event in the same way?

• A comet appears, 905: the whole sentence, with the places it names.
• Eclipses: conjunction, opposition, latitude and parallax in al-Farghani, and the observation Golius cites.
• Al-Farghani: the title page of the Latin edition, and what Golius says of the author and his time.

Read the record of the comet of 905 AH. List each place it names, then write how you would record the same sighting with an altitude and an azimuth.